Photoconductive photography



Oct. 18, 1960 E. c. GlAlMo, JR 2,956,874

PHOTOCONDUCTIVE PHOTOGRAPHY 2 Sheets-Sheet 1 Filed May l, 1956 @y1 @ifINI ENTOR.

BY f

ZY. j .519mm Oct. 18, 1960 E. c. GlAlMo, JR

PHoTocoNDucTIvE PHOTOGRAPHY 2 Sheets-Sheet 2 Filed May 1. 1956 IN V I'ENTOR. Edward f5 l il! tats 2,956,874 PHOTOCONDUCTIVE PHOTOGRAPHY rues May1, 19256, ser. No. `581,949 Tetanus; (ci. 96-15' This inventiony relatestoY photoconductive photography andy particularly, but not necessarilyexclusively, to 'mproved photoconducting methods and means' forproducing visible powder images in substantial configuration with alatent conductivity pattern in a photoconducting layer.

In one type of photoconductive photography process, a photoconductinglayer is first electrostati'cally charged by bombardment by chargedparticles as b-y the discharge fromV a corona discharge apparatus; Then,the charged surface of the photoconducting layer is exposed to alightimage incident thereon, discharging the portions irradiated' by theiight rays; while leaving the remainder of the surface in a chargedcondition, thereby forming a latent electrostatic image substantiallycorresponding to the light image. The latent electrostatic imageisdeveloped to a Visible powder image'by applying thereto a developerpowder which is held electrostatically to selected areas of the surface.The visible powder imagethusformed may be fixed directly to thephotoconducting surface or it may be transferred to another surface uponwhichs'the visible powder image'may bedesired and then fixed there- Inanother type of: photoconductive photographyprocess, a light image isprojected incident upon an uncharged photoconducting Iinsulating layerproducing therein=a latentconductivity pattern. Duringtlie decay of theconductivity pattern, the photoconducting layer is electrostaticallyvcharged by bombardment by charged particles as by the dischargefrom=acorona discharge apparatus. Electrostatic charges buildup in thelessconductingfareas ofthe. photoconducting, layerfthereby producing: a.latent electrostatic image in substantial configuration with theconductivity, pattern. The latentelectros'tatic ima'g'eis thenn.d'evelopedto a visible powder image by any. offthewell-known-methods,` as-by contacting the photoconducting insulatinglayerwith'an electroscopicipowder.

Each of these processe'sfrequires a separate step of electrostaticallycharging-the photoconductinglayer` by bombardment withycharged'particles either before or after the step` of exposure to anincident light image.l Otherproposed processes have similarlyincludedseparate charging steps by other means. In addition, each -ofthe foregoing processes requires theformation of: la latent`relectrostatic image which'must? be stored for a finite Yperiod of timeuntil the .development -of the image isfefected.

which improvedmethods omit'pthestep of electrostatica.l'l

ly charging the'photoconducting layer. l,

Another objectis to provide improved methods and means .forvdirectwconver.sionof fa latent conductivity pattern into a visiblepowder image.

In'gene'ral, the'proces'ses ofthe-invention,include es' tablishingalatent conductivity patternin'a'photocondiictlare An object of theinvention isptoprovide improved methiiatenfed oct. 1s, 1960 2 ing layer.Such pattern may be established by storing the photoconducting layer indarkness and then projecting a light image incident upon thephotoconducting layer. Then, during the decay of the conductivitypattern, establishing a unidirectional electric field through thephotoconducting layer and, With the electric field applied, contactingacross the surface of the photoconducting Vlayer a dry physical mixtureof electrostatically-attractable developer particles and magneticcarrier particles separated from the developer particles in thetriboelectric series. Developer particles deposit upon thephotoconducting layer in substantial configuration with the conductivitypattern. The electric field is preferably high atrth'e beginning and lowat the' end of the step of applying Vthe developer particles. Ifdesired, the contrast of the developed image may be varied; thedeveloped image may be reversed; and the spurious deposit of developerpowder particles in the background of the developed image may becontrolled by varying the strength and direction of the electric field.Y

The foregoing objects and other advantages will be more fully describedin the following detailed description when read in conjunction with the'accompanying drawings inwhich: l l- I l Figure l -is apartially-sectional, partially-schematic viewof-an apparatus forproducing a latent conductivity pattern in aphotoconducting layer, l g

Figure 2 is apartially-sectional, partially-schematic View of anapparatus for producing a powder image from. the latent conductivitypattern of IFigure 1 according to the invention, l

Figure 3 is a curve illustrating the conductivity of an incremental areaof a photoconductinglayer of Figures 1 and 2,during the steps oftheinvention,

vFigure 4 is a partially sectional, partially schematic view of anapparatus for, producing' visible powderjim- -agesfrom projected lightimages through the intermediate step of producing latent conductivitypatterns according to the invention, and v Y Figure 5` is a sectionalView along section lines 5-5 ofFigure 4. v

Similar reference characters are applied to similar elements throughoutthe drawings. A

ExarnpZe,`-Referring to Figure l, a photoconducting layer 23, such asphotoconducting zinc oxide dispersed in a silicone resin, and supportedupona backing 21, such as aluminum, is maintained in darkness forseveral hours. An 'image of' light' within the range of' spectralsensitivity ofthe photoconducting layer 23 incident uponthephotoconductinglayer'l causing an increase inthe' electricalconductivity of the illuminated areas thereof. This change inconductivity over portions of the-photoconducting layer is referred toas a'conductivty pattern. j The conductivity pattern -is latent'and issubstantially in the same configuration as the incident light image.V Asshown in Figure 1, a photographic transparency (positive) 24 `is placedupon the photoconducting layer`23 and exposed =of'biasinggvoltageattached to thepole piece 27.'

The magnetic pole piece.v 27 comprises an iron'. with a permanentlymagnetized pole at one end thereof.

The magnetic pole produces an external magnetic field which attracts andholds the mass of developer mix 29. The attracted mass of developer mixis loosely held and easily deformable to the contours of a surface withwhich it may be in contact. At the other end of the iron bar, a threadedhole and screw provides a connection means 37 for a source of biasingvoltage to the developer brush.

A battery 31 or other source of biasing voltage is connected to themagnet 27 through the connection means 37, a double-pole, double-throwreversing switch 33 and a potentiometer 35. Thus, the applied biasingvoltage may be changed in polarity and varied in magnitude.

The switch 33 and the potentiometer 35 are adjusted such that about-l-700 volts with respect to ground is applied to the magnet 27, and thedeveloper mix 29 of the brush is contacted across the surface of thelayer 23. During the period of contact, a unidirectional electric fieldappears between the mass of developer mix 29 and the backing 21 due tothe biasing voltage applied between the brush and the backing 21.Developer powder particles 67 from the mass of developer mix 29 depositon the less conducting areas of the photoconducting layer 23 producing adirect visible powder image thereon.

The visible powder image produced by the abovedescribed procedure has avery high contrast characteristic and a minimum amount of spuriousdeposit in the background areas. This set of characteristics isconsidered ideal for line drawings and line prints. By reducing thebiasing voltage and therefore the unidirectional field, the contrastbetween the dark and light areas of the image is reduced, making itpossible to obtain any desired contrast characteristic over a very widerange of contrast values. A preferred range of positive voltage is about+500 to +1500 volts. Often, a moderate amount of developer powderdeposits spuriously in the background areas of the Visible powder image.Such spurious deposit may be reduced to a minimum value by adjusting thebias on the magnetic brush.

If the switch 33 is reversed and an increasing voltage applied to themagnet 27, a value is reached where there is developed a reverse visiblepowder image; that is, the powder deposits in the more conducting areasof the photoconducting layer. The contrast of the reverse Visible powderimage increases and the amount of spurious deposit in the backgroundareas of the reverse image decreases as the negative voltage isincreased in magnitude. A preferred range of negative voltage is about-500 to 1500 volts.

According to the invention, a direct or reverse visible powder image maybe obtained from the same latent conductivity pattern. A direct visiblepowder image is a developed image wherein the developed areas correspondto the non-illuminated areas of the original light image. A reversevisible powder image is a developed image wherein the developed areascorrespond to the lighted areas of the original light image. Byadjustment of the biasing voltage, any desired contrast characteristieover a wide range may be obtained for either the direct or the reverseimage. Thus, one may simply and quickly adjust an electrostatic printingapparatus to produce line prints of high contrast value, continuous toneprints of intermediate contrast value and, in each case, the print maybe direct or reverse.

Any photoconducting layer usable in electrophotography maybe used in theinvention. Some of the useful photoconducting layers are described by C.I. Young and H., G. Greig in Electrofax-Direct ElectrophotographyPrlnting on Paper, RCA Review, December 1954, volume 15, No. 4, pages469 to 484; by E. Wainer, Phosphor Type Photoconducting Coatings forContinuous Tone Electrostatic Electrophotography, PhotographicEnglneering, volume 3, No. 1, 1952, pages 12 to 22; and by A. Middletonin U.S. Patent No. 2,663,636, issued December 23, 1953.

Some suitable photoconducting coatings are photoconducting zinc oxide(AZO 33 marketed by the New Jersey Zinc Co., Palmerton, Pa.) dispersedin a silicone resin, photoconducting zinc selenide (Mallinkredt No. 8856marketed by the Mallinkrodt Chemical Works, New York, N.Y.) dispersed ina silicone resin, photoconducting zinc sulfide (Cryptone ZS 800 marketedby the New Jersey Zinc Co., Palmerton, Pa.), andpanchromaticallysensitive zinc oxide prepared according to either U.S.Patent No. 2,727,807 or 2,727,808 to S. M. Thomsen dispersed in asilicone resin. In place of a silicone resin one may substitute apolystyrene resin, a polyvinyl acetate resin, a polyvinyl-chlorideacetate resin, carnauba wax or other electrically-insulating,film-forming vehicle.

A preferred composition may be prepared by intimately mixing 100 gramsof a photoconducting zinc oxide, such as Florence Green Seal No. 8marketed by the New Jersey Zinc Company, Palmerton, Pa. with 65 grams ofa 60% solution of a silicone resin in xylene (such as GE SR-82 marketedby the General Electric Co., Silicone Products Division, Waterford,N.Y.), and 85 grams of toluene.

After ball-milling to obtain a smooth, uniform consistency, the mixtureis coated on the surface of a paper web and dried. Any standard coatingtechnique may be used such as flowing, spraying dipping, spin-coating,or brushing on.

in addition to the vehicle-bound type coatings described, one may usecoatings having no vehicle such as sulfur, anthracene or selenium. Whereselenium is used, the magnetically responsive carrier particles shouldbe non-conducting such as a ferrospinel powder or a resin-coated ironpowder.

An essential part of the developing procedure is that it must be carriedout during the decay of conductivity of the photoconducting layer.Referring to Figure 3, there is shown a plot of conductivity in anincremental area of the photoconducting layer 23 of Figures 1 and 2 withrespect to time. The photo-conducting layer is stored in darkness forthe period 0-1 and reaches a steady state of minimum conductivity asshown by .1X-B. Light is turned on during the period 1-2, which is theperiod of exposure to the light image, during which the conductivityrises to a maximum value as shown by B-C. After the light is turned offat 2, the conductivity begins to fall to a minimum value during theperiod 2-3 as shown by C--D. It is during the interval 2-3, thatdevelopment of the latent conductivity pattern must take place.Development is preferably carried out as soon as possible in order totake advantage of larger differences in conductivity. Thephotoconducting coating of the example has a very long period of decayand development may take place during a period of at least 16 hoursafter exposure to the light image. However, development preferably takesplace within a period of 30 minutes after exposure to the light image.

The developer powder particles of the deevloper mix may be chosen from alarge class of materials, for example; zinc, copper, carbon, sulphur,gum copal, gum sandarac, nylon, polystyrene, sealing wax and othernatural or synthetic resins of mixtures thereof. The developer powderparticles may be coated with a thin layer of a material for the purposeof modifying the physical or electrical properties of the developerpowder. It is preferred, however, to use a pigmented thermoplasticsynthetic resin.

A preferred developer powder of the example may be prepared as follows:a mixture comprising 200 grams of 200 mesh Piccolastic resin 4358 (anelastic thermoplastic resin composed of polymers of styrene, substitutedstyrene and its homologs) marketed by the Pennsylvania IndustrialCompany, Clairton, Pa., and 12 grams of Carbon Black G marketed by theEimer and Amend Co., New York, N.Y., are thoroughly mixed in a stainlesssteel beaker at about 200 C. The mixing and heating should aesas'ca bedoneziri'as short..a:time.aspossible... Thentelt is poured' uponV abrass tray. and. allowed. tocool. and harden:v T.'l1e-.hardened` mix isthenbroken upl and ballmilled for about 20 hours.. Thepowder. is`screened through 2112.00. meshscreenand'is. then. ready for use asadeveloper4 powder. This`- powder takesA on. a positive electrostaticcharge7 whenmixed with: iron.. powder. It therefore will develop.` themore.' insulating areas.. of a latent conductivity-patternwith.-apositive biasing voltage appliedto the magnetic-brush.

The magnetic carrier particles of the. developer mix may be chosenfrom.a large class of .powderedtm'ag netically-attractable. materials such asiron, steel, aldoys of aluminum, nickel. and cobalt and other. magneticmaterials.

A preferredcarrier material-:for thedeveloper mix of the. examplelconsists of alcoholzed iron, that is, iron particles free from greaseand other` impurities soluble in alcohol: These iron particlesrarepreferably relatively smallin size,v.being intheir largestdimensionabout 0.002 to 0.008 inch. Satisfactory results are alsoobtained using. acarrier. consisting, of iron.particles..of. a somewhatwider range. of. sizes-.up to. about 0.001. to. 0.02.0 inch. It ispreferred to utilize a permanent bar magnetv for providing themagneticflield. for` maintaining .thedeveloper mix in a loose-massi. However,other structures may be used, such.as.electromagnets:or othermagneticiield producing means:. Similarly. thebiasing: voltage. may` beapplied to the mass of developer mix.. of.' the brush as shown in Figure2 or may be applied to the backing plate 44afwith=tle developermix-.connected .toeground, as subsequently described in F igure. 4.

A positive or a negative voltage may be used to bias thefma'gne'tic-brush-so long-as-there is `no electrical breakdown in thebrusl-iorinthe l photoconductng" layer( 23 upon whichI the latentconductivitypattern resides;` It ispreferred; however, to-useavoltage'rbetweenl--ISOO to 500 and-+500 to `[-15500 voltsfDlC.Tlieoptim-um rangeA ofvoltages dependsupon the'thicknessandi theelectrical propertieswofv thephotoconducting layer Z3.

Developmentoccurs instantaneously with i1500 volts betweenthe brush 291andthe-backing 213 In order to remove spurious deposits of developerpowder in the background areas, the voltage may be reduced to about 500volts in the same polarity after the initial deposit of developerpowder. Various rates of lowering the voltage may be used so long as thehigher voltage preceded the lower voltage.

Referring to Figures 4 and 5, the improved methods and means of theinvention may be embodied in an improved continuous electrostaticprinting process and apparatus. A continuous web comprising a paperbacking 51 having on one surface thereof a photoconductng insulatingcoating S3 comprising a powdered photoconductor, such as zinc oxide,dispersed in an electricallyinsulating, film-forming vehicle is unwoundfrom a roll 5S.

The continuous web next passes to a station where a light image isproduced upon the surface of the photoconducting coating 53, forexample, by projection from a photographic transparency image by meansof a projector 57. The photoconductng insulating layer 53 now hastherein a latent conductivity pattern substantially corresponding to thelight image which was projected thereon.

The continuous web next advances to a station where the latentconductivity pattern is developed by the method of the invention bycontact with a first magnetic brush. A grounded rotary pole piece 63a ofa magnetic structure is provided with spaced parallel inclinedelliptical discs 61a facing the photoconductng coating 53. A magneticfield is maintained between a fixed magnetic pole piece 45a spaced fromthe elliptical discs 61a and on the opposite side of the web throughmagnetic pieces 47a and 49a and through the gap therebetween. A resereG? Y voir 65h holds a quantity of' developer iii Contact with the discs6321. Ashield 4h`t1lo'cated'A betweenY the fixed polepiece 45a and theweb is maintainedin contactwitl. or closelir spacedl behind the paperbacking 51 and'is connectedto avoiltage source 31a through adouble-pole, double-throw reversing switchV 33a and a potentiometer 35a.TheV switch 33a andthe potentiometer 35a are -adjustedto provide thedesired' biasing voltage upon the shield 44`for example, .a -voltage of.-l- 1500 volts. As the rotary pole piece 63arotates in a' clockwisedirection as viewed in Figure 4, developer' mix forms on the peripheryofthe idiscs-.61a in brush-like filaments 69a and iscarried'upwardly andswept acrossltheV surface of the photoconductnginsulatingcoating'53`passingL the station. Developer powder particles 73deposit upon areas previously unilluminated producing a reversevisiblepowder image on. th'e photoconductng coating 53 in substantiaiVcorlg'uration with. the latent conductivity pattern. According' to theinvention, direct or reverse visible. powder images may be produced' andthe background'colo-r valueV and the contrast value-ofthe visible imagemay be varied byadjustingthe biasing voltage to the shield l44 as`previously' described in Figure 2.

The continuous web now passes to a station where thephotoconductingcoating 53l is contacted with a second magnetic brush.They second magnetic brushv has the same structure and operates in thesame manner as the first magnetic brush* except that the' biasingvoltage on the shield-44]; ismaintained at about +500 volts. Thefunction'of'the'second'magnetic brush is to'clear away any spuriousdeposit of`-"devel'oper powder without disturbing the powder image v67.p

The continuous web -bearingthevisible powderv image73ftli'ereonnowpasses to -a station where the visible image is 'xedto'the-photoconductive coating 53: For this purpose aradiantheaterfcomprisingt axresis-tance wire 79 connected to yalvoltage source75ith`rough'a potentiometer 77, The resistance wire 79v is maintained inclosely spaced relationship withJ the visiblelpowder' image. Heatradiated.. from` thef wire.5 79f softens. the' thermoplastic resi-motthedevelope'r.- powder causing itzto adhere` to `the;vphotocond'u'ctingf' coating.. 53: The visiblel powder image may be fixedto the photoconductng layer 53 by other means for example, by sprayingwith an adhesive or by coating with a softener for either thephotoconductor 53 or the developer powder particles. The visible powderimage may also be transferred to another surface and fixed thereon byany convenient means. The continuous web bearing the fixed visible imageis now wound upon the roll 56. The continuous web of course, may be cutinto convenient lengths and stacked in piles or utilized directly.

There have been described improved methods and means of photoconductivephotography including improved methods and means for developing latentconductivity patterns. There have also been described methods and meansfor producing direct or reverse visible images and for controlling thecontrast value of the visible powder image developed according to animproved photoconductive photography process.

What is claimed is:

l. An electrostatic printing process comprising the steps of producing aconductivity pattern in a photoconductng insulating layer consisting ofa finely-divided photoconductor dispersed in an electrically-insulatingvehicle by exposing `said layer to a light image before any electricalfield s established through said layer, then magnetically transportingacross said conductivity pattern a dry physical mixture of loose,movable particles of electrostatically-attractable powder 'and separateelectrically conductive magnetically-attractable carrier particles bysubjecting said magnetically `attractable particles to a magnetic fieldbrought across said conductivity pattern, said carrier particles andpowder particles having a triboelectric relationship of oppositepolarity, the powder particles thereby being electrostatically-chargedthrough triboelectric action by contact with the carrier particles toadhere electrostatically to the surface of the carrier particles, andduring said transporting step applying a biasing voltage to saidelectrically conducting particles with respect to the opposite surfaceof said layer to establish through said layer a unidirectional electriceld and to cause powder particles to become attracted to said insulatinglayer and to adhere thereon in substantial contiguration with saidconductivity pattern.

2. An electrostatic printing process comprising the steps of producing aconductivity pattern in a photoconductiug insulating layer consisting ofa finely-divided photoconductor dispersed in an electrically-insulatingvehicle by exposing said layer to a light image before any electricalfield is established through said layer then magnetically transportingacross said conductivity pattern a dry mixture of loose, movableparticles of electrostatically-attractable powder and separateelectrically conductive magnetically-attractable carrier particles bysubjecting said magnetically-attractable particles to a magnetic eldbrought across said conductivity pattern, said carrier particles andpowder particles having a triboelectric relationship of oppositepolarity, the powder particles thereby being electrostatically-chargedthrough triboelectric action by contact with the carrier particles toadhere electrostatically to the surface of the carrier particles,spacing an electrode from said mixture and on the side of saidinsulating layer opposite to said mixture, and applying a biasingvoltage between said electrode and said magnetically `attractableparticles during said transporting step to establish a unidirectionalelectric eld through said insulating layer and to cause powder particlesto become attracted to said insulating surface and to adhere thereon insubstantial configuration with said conductivity pattern.

3. An electrostatic printing process comprising the steps of producing aconductivity pattern in a photoconducting insulating layer consisting ofa nely-divided photoconductor dispersed in an electrically-insulatingvehicle by exposing said layer to a light image before any electricaleld is established through said layer, then magnetically transportingacross said conductivity pattern a dry mixture of loose, movableparticles of electrostatically-attract- CIK able powder and separateelectrically-conductive magneticallyattractable carrier particles bysubjecting said magnetically-attractable particles to a magnetic fieldbrought across said conductivity pattern, said carrier particles andpowder particles having a triboelectric relationship of oppositepolarity, the powder particles thereby being electrostatically-chargcdthrough triboelectric action by contact with the carrier particles toadhere electrostatically to the surface of the carrier particles,spacing an electrode from said mixture and on the side of saidinsulating layer opposite to said mixture, applying a biasing voltagebetween said electrode and said magneticallyattractable particles duringsaid transporting step to establish a unidirectional electric eldthrough said insulating layer and to cause powder particles to becomeattracted to said insulating surface and to adhere thereon insubstantial configuration with said conductivity pattern, and thenrepeating said contacting step with a lower biasing voltage applied.

4. The process of claim 2 wherein said insulating layer comprisesphotoconducting zinc oxide dispersed in a silicone resin.

5. The process of claim 3 wherein said insulating layer comprisesphotoconducting zinc oxide dispersed in a silicone resin.

6. A method according to claim 2 wherein said biasing voltage is between500 and 1500 volts.

7. A method according to claim 3 wherein said biasing voltage is about1500 volts and said lower biasing voltage is about 500 volts.

References Cited in the le of this patent UNITED STATES PATENTS1,874,912 Scott Dec. 16, 1930 2,408,143 Huebner Sept. 24, 1946 2,618,551Walkup Nov. 18, 1952 2,633,796 Pethick Apr. 7, 1953 2,784,109 WalkupMar. 5, 1957 2,803,177 Lowrie Aug. 20, 1957 OTHER REFERENCES RCA Review,December v1954, pp. 469 to 484.

b ver H "br UNITED STATES PATENT oEEICE CERTIFICATE OE CORRECTION PatentNo. 2956q874 October 18 1960 n Edward Charles Giaimoq Jr.,

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below. I

Column 8, list of references etedl under the heading "UNITED STATESPATENTS"v add the following:

same column 8 under the heading "OTHER REFERENCES'HI add the following:

Weiner, Photographic Engineering Vol, 8v Non ly ppc n Signed and sealedthis 18th day of April l96l.,

(SEAL) Attest:

` ERNEST VIL7 SWIDER DAVID L.,l LADD Attestlng Oicer Commissioner ofPatents

1. AN ELECTROSTATIC PRINTING PROCESS COMPRISING THE STEPS OF PRODUCTINGA CONDUCTIVITY PATTERN IN A PHOTOCONDUCTING INSULATING LAYER CONSISTINGOF A FINELY-DIVIDED PHOTOCONDUCTOR DISPERSED IN ANELECTRICALLY-INSULATING VEHICLE BY EXPOSING SAID LAYER TO A LIGHT IMAGEBEFORE ANY ELECTRICAL FIELD IS ESTABLISHED THROUGH SAID LAYER, THENMAGNETICALLY TRANSPORTING ACROSS SAID CONDUCTIVITY PATTERN A DRYPHYSICAL MIXTURE OF LOOSE, MOVABLE PARTICLES OFELECTROSTATICALLY-ATTRACTABLE POWDER AND SEPARATE ELECTRICALLYCONDUCTIVE MAGNETICALLY-ATTRACTABLE CARRIER PARTICLES BY SUBJECTING SAIDMAGNETICALLY ATTRACTABLE PARTICLES TO A MAGNETIC FIELD BROUGHT ACROSSSAID CONDUCTIVITY PATTERN, SAID CARRIER PARTICLES AND POWDER PARTICLESHAVING A TRIBOELECTRIC RELATIONSHIP OF OPPOSITE POLARITY, THE POWDERPARTICLES THEREBY BEING ELECTROSTATICALLY-CHARGED THROUGH TRIBOELECTRICACTION BY CONTACT WITH THE CARRIER PARTICLES TO ADHERE ELECTROSTATICALLYTO THE SURFACE OF THE CARRIER PARTICLES, AND DURING SAID TRANSPORTINGSTEP APPLYING A BIASING VOLTAGE TO SAID ELECTRICALLY CONDUCTINGPARTICLES WITH RESPECT TO THE OPPOSITE SURFACE OF SAID LAYER TOESTABLISH THROUGH SAID LAYER A UNIDIRECTIONAL ELECTRIC FIELD AND TOCAUSE POWDER PARTICLES TO BECOME ATTRACTED TO SAID INSULATING LAYER ANDTO ADHERE THEREON IN SUBSTANTIAL CONFIGURATION WITH SAID CONDUCTIVITYPATTERN.